Multiple wireless Tags can be interrogated by sending a code from an interrogating transmitter (e.g., a Reader) and having information transmitted by the Tag in response. This is commonly accomplished by having the Tag listen for an interrogation message and for it to respond with a unique serial number and/or other information. The Tags typically have limited power available for transmitting data wirelessly to the Reader. It is desirable to extend the range of wireless Tags so that it is not necessary to bring each Tag close to a Reader for reading. However, when the range of the reading system is extended, many Tags will be within the range of the interrogating system so that their replies may corrupt each other.
Current implementations of radio frequency (RF) Tags require considerable logic to handle interface protocols and anti-collision problems that occur when multiple Tags within the range of a Reader all attempt to reply to an interrogating message. For example, current integrated circuits that are used in RF Tags require nearly 3,000 logic gates to handle an interface protocol and to handle anti-collision protocols. This considerable size required by an integrated circuit increases the cost of the RF Tag and thus makes it less likely for such a Tag to be more commonly used. Prior art attempts to avoid collisions when reading multiple RF Tags are described in U.S. Pat. Nos. 5,266,925, 5,883,582 and 6,072,801. However, these prior art approaches provide inefficient solutions for avoiding collisions when reading multiple RF Tags.